Conventional sensors used to determine a chemical or biological characteristic of a target compound rely measuring emission signals emitted by the target compound in response to an excitation signal incident on the target compound. In some cases, a fluorophore is added the target compound to increase the intensity of the emission signal. A reaction of the target compound to the fluorophore may be characterized by an optical emission signal generated by target compound in response to the excitation signal. The characteristics of the target compound can then be determined based on the emission signal. In other cases, the emission signal may be an attenuated version of excitation signal received after passing through or being reflected by the target compound. One example of an attenuation-based measurement involves transmitting the excitation signal into a blood sample and determining the oxygen level of the blood sample based on the intensity of the emission signal, i.e., the optical signal that emerges from the sample. This type of optical measurement focuses on how the target compound attenuates the emission signal. In either case, the intensity of the optical signal emitted by the target compound is used to identify the characteristics of the target compound.
One disadvantage of intensity measurements is that they are susceptible to aging of the detector electronics, varying ambient conditions, and environmental noise. The resulting fluctuations in the detected intensity of the emission signal may produce inaccurate results. Further, each time the excitation signal is introduced into the target compound, the optical characteristics of the target compound can change, e.g., due to bleaching of the target compound or fluorophore. This degradation may cause the intensity of the emission signal to vary so that it is no longer able to provide accurate information regarding the characteristics of the target compound.
Another way to characterize a target compound is to measure an excited state lifetime of the emission signal. This method also involves using an excitation signal to excite fluorescent receptors in the target compound, which can include an inherent fluorophore in the target compound or a fluorophore introduced into the target compound, and detecting an emission signal emitted by the excited fluorescent receptors. The excited state lifetime of the fluorescent receptor is then determined based on the behavior of the emission signal and used to identify the characteristics of the target compound. Unlike the absolute intensity of the emission signal, the excited state lifetime of the target compound is not impacted by changes in the detector electronics, ambient conditions, or environmental noise. The excited state lifetime is also unaffected by bleaching. Thus, using an excited state lifetime to characterize a target compound can provide more accurate results as compared to simple optical signal intensity-based measurements.
Conventional excited state lifetime measurements monitor a decay in the emission signal as the excitation signal interacts with the target compound, with the rate of decay in the emission signal being indicative of the characteristics of the target compound. For example, one type of characteristic of the target compound may be identified for a relatively long excited state lifetime (e.g., ≥100 ns) as determined from a correspondingly slower decay rate of the emission signal. Another characteristic of the target compound may be indicated by a relatively short excited state lifetime, e.g., ≤50 ns.
One problem with conventional excited state lifetime monitoring systems is that they are limited to monitoring relatively long excited state lifetimes, which restricts the type of fluorescent receptors that can be monitored. Monitoring the duration of the excited state lifetime also requires electronics that compute at extremely fast rates to capture the excited state lifetime of the fluorophore before the excited state lifetime expires. Such electronics significantly increase the cost of these systems as well as the space that the systems occupy. Moreover, these large and costly systems are still are unable to detect excited state lifetimes below about 100 ns. Thus, conventional excited state lifetime monitoring systems are limited in the types of fluorescent receptors that can be used and in turn are limited in the types of chemical or biological characteristics that can be identified.
Thus, there is a need for improved apparatuses, methods, and computer program products for performing optical analysis of target compounds, and specifically, that accurately determine excited state lifetimes of the target compounds, particularly those having short durations.